This invention relates generally to a duplicating machine and, more particularly, to a machine for duplicating images on both sides of copy sheets, hereinafter sometimes referred to as "duplexing".
Duplicating machines are available for the production of copies with images formed on one side of the copy sheets. Such equipment can be reliably operated at highly satisfactory production rates. Because of the advantages of duplexing in savings of the amount of paper employed, savings in the space occupied by the copies produced, and savings in production time and equipment costs, it is desirable to provide apparatus for imaging both sides of a copy sheet.
Duplexing often is effected by duplicating machines employing a single printing couple to thereby provide a compact unit that may be utilized in small work areas and conserve the amount of floor space required in which to operate the equipment. However, single printing couples for duplex printing require relatively large and expensive master cylinders, blanket cylinders and impression cylinders because of the multiple images required on a single cylinder. Sometimes the cost is prohibitive. In addition, relatively complex gripper mechanisms are required on the impression cylinder, as well as complex mechanisms for handling sheets released from the impression cylinder and for re-feeding the sheets back to the gripper mechanisms on the cylinder.
Consequently, it is desirable to utilize plural printing couples employing less expensive cylinders and gripper mechanisms where the work area or floor space in which the machine is to be utilized is not a premium.
There have been various approaches in the printing and duplicating field for printing a copy sheet on a first side by a first printing couple and then on the opposite side by a second printing couple. In some instances, the printing couples are arranged in straight-line, tandem. Of course, when the printing couples are arranged in tandem, the overall size of the combined unit is extended considerably thereby requiring additional floor space.
Another approach to duplex printing has been to provide a sheet handling mechanism for passing a copy sheet through a first printing couple for imaging one side of the sheet in a first direction and advancing the sheet in a second, substantially normal direction to a second printing couple. As the sheet changes direction, it is inverted by a turn-over device and simultaneously directed to the second printing couple for imaging the opposite side of the sheet. Such machines are more compact than the tandem arrangement because of the L-shaped configuration of the sheet advancing paths. However, such an arrangement still is considerably more expensive and complex than the simple tandemly arranged printing apparatus.
An area of concentration in reducing the length or overall size of tandemly arranged duplicating machines is in the mechanism for reversing the sheets after receiving the first image at the first printing couple and inverting the sheets for receiving the second image at the second printing couple. Heretofore, mechanical means such as rakes employing mechanical fingers have been utilized to engage the trail ends of copy sheets travelling through the tandem machine, along with other mechanical mechanism, to reverse and invert the sheets. However, such mechanical means have a tendency to break, particularly breakage of the rake fingers, or to become deformed which results in sheet jamming. In addition, complex and expensive mechanical linkage or gear trains are required to drive the sheet handling mechanism in synchronization with the issuing of sheets from the first printing couple.
This invention is directed to solving the sheet handling and reversing problems of the prior art, in duplexing machines of the tandem type employing printing couples in a straight-line arrangement.